Miniaturized satellites, such as cubesats, are increasingly being used for information transmission and space exploration. Compared to conventional satellites, miniaturized satellites reduce the largest cost associated with space exploration: the cost of spacecraft deployment. With proper propulsion and control, these miniaturized satellites have the capability to explore Earth, asteroids and other planets with significantly lower cost and greater maneuverability.
Conventional satellites typically incorporate chemical thrusters that utilize liquid propellants such as Hydrazine and Aerozine-50 for propulsion. However, the exhaust velocity of such chemical thrusters is limited by the inherent specific energy released by combustion and thus chemical propulsion uses propellant more inefficiently than electric propulsion. In addition, the propellant reservoirs and feed systems for chemical thrusters are large and heavy, rendering them incompatible with miniaturized satellite and small spacecraft propulsion.
A potential solution to the complexities of chemical propulsion is the electrospray thruster, which is a form of electric propulsion that creates thrust from liquid propellants by ejecting and accelerating charged particles. In these thrusters the charged ions are accelerated by electrostatic forces. Electrospray thrusters utilizing field-emission electric propulsion (FEEP) are more efficient than conventional electrostatic ion or Hall effect thrusters. In addition, FEEP thrusters have the potential to scale down or up in size, mass, and thrust range to propel miniaturized or conventional sized spacecraft. Because of their low thrust level capability, FEEP thrusters are most useful for micro-newton to milli-newton propulsion applications for high velocities on small spacecraft and for precision pointing small and large spacecraft.
FEEP electrospray thrusters create high exhaust velocities on the order of thousands of meters per second utilizing high voltages. Electrospray thruster devices essentially consist of an emitter, an extractor electrode, a heater, an isolator, a propellant reservoir, and, in some instances, an accelerator electrode. In many of these systems the emitter is an externally wetted needle or an internally wetted capillary tube. A potential difference of the order of 2-10 kV is applied to generate a strong electric field at the tip of the emitter. Voltages just above a threshold draw the liquid propellant into a Taylor cone and extract charged particles at the apex of the cone. The charged particles are then accelerated to high velocities on the order of tens of kilometers per second by the applied electric field. To balance the charge loss, a separate negatively charged particle source is used to neutralize the ions and spacecraft charge imbalance. This process of creating and accelerating charged particles is very efficient, with reported beam efficiencies of greater than 90%.
Several different liquid propellants, including liquid metals, may be used to wet the FEEP thruster emitters to generate charged droplet beams. Typical liquid metal ion sources (LMIS) may include gallium, indium, gold and alkali metals or alloys. In some cases the propellant is stored as a solid, melted to flow, and then pulled up the emitter tip by capillary forces. These propellants can be used in both ion and droplet emission modes for electric thrusters operating at high and low specific impulse.